It's French for répondez s'il vous plaît, meaning “reply please” or "please respond" (Wikipedia).
When receiving an invitation, it's not only courteous to RSVP, but in many cases required. Large events like weddings, for example, depend on their guests to respond. The host did their part. The guest must do theirs as well.
But, sometimes we procrastinate.
I know, for me, I'll throw an invite in a pile of 'to do' papers and it simply gets lost in the shuffle of things to get done. I will either label it as 'low priority' or just completely overlook it altogether. If I'm not careful, I may never RSVP at all. The wedding will still take place. But, a seat will not be saved for me.
We have been INVITED.
There's a Party. A Banquet that is going to take place. Soon. Yes, very soon.
The invites have gone out. The Host has done His part. But, He waits now for our RSVP.
But, some will procrastinate.
They will throw it in the pile of 'things to do before I die'. They label it 'low priority' or overlook it altogether. And, if they are not careful, their 'time to go' will come upon them like a thief in the night. The Party will still take place. But, a seat will not be saved for them.